1. Field of the Invention
The present invention relates to a method and apparatus for detecting the gap of a liquid-crystal panel, in particular, a reflective liquid-crystal panel.
2. Description of the Related Art
Liquid-crystal panels feature small weight and thickness and a low power consumption. For this reason, liquid-crystal panels have been widely used in monitors, displays, and the like. In particular, reflective liquid-crystal panels having a reflective mirror on one substrate use light illuminating the liquid-crystal panel from the outside as an illumination light source and, therefore, require no light source unit. For this reason, reflective liquid-crystal panels have been used as displays for portable information terminals or cellular phones which have an especially low power consumption.
In the reflective liquid-crystal panels that are presently used, liquid crystal molecules are oriented almost parallel on both substrates, but the orientation direction of liquid crystal molecules between the substrates is twisted. Furthermore, even among the reflective liquid-crystal panels of a type in which the orientation direction of liquid crystal molecules between the substrates is twisted, the reflective super twisted nematic liquid-crystal panels (reflective STN liquid-crystal panels) or reflective twisted nematic liquid-crystal panels (reflective TN liquid-crystal panels) are mainly used. In the reflective STN liquid-crystal panels, the twisting angle of orientation direction of liquid crystal molecules is no less than about 180°. In the reflective TN liquid-crystal panels this angle is no more than about 120°. A general name of reflective liquid-crystal panels will be given hereinbelow to the reflective STN liquid-crystal panels and reflective TN liquid-crystal panels.
The display performance of reflective liquid-crystal panels depends of the gap between the substrates, refractive index of liquid crystals, twisting angle of orientation, pretilt angle (angle at which the liquid crystal molecules rise with respect to the flat surface of substrates), and the like. The control of gap is especially important in the manufacture of reflective liquid-crystal panels. Such a control is not limited to reflective liquid-crystal panels and is also important in the manufacture of transmission liquid-crystal panels. In the transmission liquid-crystal panels, illumination with light from a light source is conducted from the rear surface of the liquid-crystal panel.
Various methods have been used for detecting the gap of reflective liquid-crystal panels.
For example, a method was suggested for determining the gap from the pattern of interference stripes observed in the reflection spectrum in a near-IR region (Japanese Liquid Crystal Society, Preprints of Reports presented at the 2000 Meeting, PCb03, p. 341).
However, the method for detecting the gap of liquid-crystal panels from the pattern of interference stripes does not take into account the presence of a large number of interfaces making contribution to the interference, the necessity of refractive index dispersion in the liquid crystal with respect to light in the near-red range, and a twisted orientation of liquid crystal molecules. For this reason, the calibration curve (a curve representing the relationship for correcting the numerical values obtained by measurements to actual gap values) has to be plotted.
Furthermore, an attempt was also made to detect the gap of liquid-crystal panels form the retardation Δnd (Δn is the refractive index anisotropy of a liquid-crystal and d is the thickness of a liquid-crystal layer).
However, under the orientation conditions of liquid crystal employed in the reflective TN liquid-crystal panels, the retardation is constant, regardless of the gap. For this reason, a method for detecting the gap of liquid-crystal panels from the retardation is difficult to use for detecting the gap of reflective TN liquid-crystal panels.
The conventional methods employed complex apparatus and involved troublesome operations or processing. In particular, the detection time was long.